WIP: add bbox endpoint for forms, dataviews, and merged datasets

[FrankApiyo]

Summary

Adds GET /api/v1/forms/{id}/bbox, /api/v1/dataviews/{id}/bbox, and /api/v1/merged-datasets/{id}/bbox.

Each returns {"bbox": [min_lng, min_lat, max_lng, max_lat] | null} — the extent of geolocated submissions (null when the target has no geolocated rows, so callers can fall back to a default view cleanly).

Motivation

Zonkey's tile-based map view (Martin + form_tiles()) cannot compute bounds client-side since features are paged per MVT tile. A dedicated endpoint lets the frontend fit the viewport on first load without fetching the full submission set.

Design

• onadata/libs/utils/bbox_tools.py — shared compute_instance_bbox(xform_ids, dataview=None) helper using Django GIS Extent aggregate
• Filter shape mirrors the form_tiles() PostGIS function (non-deleted, geom non-null, optional dataview query filter) so the fit matches the data Martin serves
• Three viewset @action methods (XFormViewSet, DataViewViewSet, MergedXFormViewSet) wrap the helper; permissions inherit from each viewset's existing class

Test plan

• test_bbox_returns_extent_of_geolocated_submissions — XForm with the gps fixture returns a valid 4-tuple bbox
• test_bbox_null_when_no_geolocated_submissions — returns {"bbox": null} for forms without geoms
• DataView + MergedXForm bbox actions return the correct shape (null-case covered; the helper is exercised via the XForm extent test)
• Run full onadata test suite in CI
• Verify against a live form on stage once deployed

Consumers

• Zonkey tile map view (TileMapView) → useFormBbox hook → fitBounds on map load. Work in progress on the martin branch of zonkey.

Why WIP

Marking WIP pending:

• Discussion on whether the bbox_tools helper should live in libs/utils/ or somewhere closer to the logger models
• Decision on whether to cache the endpoint (bbox changes slowly; 5-min Redis cache would be cheap)
• Review of whether the null-return for empty datasets is preferable to 404 (I went with null to make the frontend branch simpler)

Replaces #3066 (rebased onto main, branch renamed from wip/bbox-endpoint to bbox-endpoint).